The present invention relates generally to the field of visual display systems for such purposes as, for example, entertainment, training, simulation, vehicle status displays, virtual reality, machine maintenance, and the like. More particularly, the present invention relates to collimating generated image light to focus it into the line-of-sight of an observer so that it appears to be at or near infinity, yet providing high transmission of images incident thereupon.
Optical collimating apparatus have been known for some time. One example of image light collimation is described in U.S. Pat. No. Re 27,356, reissued May 9, 1972, which uses a single spherically curved combining mirror as an image forming element. A primary image is directed at the convex side of the mirror, which transmits the image to a birefringent beam splitter array, positioned on the concave side of the mirror. The array reflects the image back to the spherical mirror, which collimates the image for viewing by an observer. Several polarizing filters in the light path selectively direct that part of the primary image which returns to the spherical mirror. However, the filtering and reflection of the primary image will tend to successively reduce the intensity of the image. The result is an ultimate transmission in the neighborhood of 0.5 to 1.0 percent of the original intensity of the primary image. Thus, the image source must have sufficient power to produce an image of acceptable brightness when viewed.
Another collimating system, described in U.S. Pat. No. 3,940,203, issued Feb. 24, 1976 is a variation on the previously discussed U.S. Pat. No. Re 27,356 in that the spherical mirror is replaced with a reflection-type holographic analog of a spherical mirror. Again, a relatively large number of reflections and transmissions are employed to properly control the image light reaching the observer. By utilizing improved reflection and transmission characteristics of the holographic element, efficiency on the order of 6 to 10 percent of an image's original intensity is achieved.
Collimation apparatus will also be found in head up display (HUD) systems in cooperative association with a combiner element to superimpose a generated image on a forward field of view. In many configurations, the combiner element and the collimation apparatus require multiple filtering and reflecting of the generated image which, as indicted above, can influence the transmissivity and have a negative effect on the brightness of the image. An example of a modern system of this type is found in U.S. Pat. No. 4,859,031, issued Aug. 22, 1989. Described therein is a system that utilizes to good effect the properties of a cholesteric liquid crystal as an efficient reflector to a semi-reflective concave mirror and then as an efficient transmitter of the collimated image to an observer. The image collimating apparatus is positioned between the image source and the observer, and the generated images are first polarized to a particular rotary sense of circular polarization. So polarized, the image is transmitted to, and then reflected by, a semi-reflective mirror. This arrangement improves transmission efficiency over prior art to at best 12.5 percent of the original image intensity, but the polarization and the multiple reflection path of the semi-reflective concave mirror can still influence image intensity negatively.
Thus, there is a present need in the art for an optical collimating apparatus with improved transmissivity of images. Further, since application of such optical apparatus may include head-mounted display systems or other applications where size and weight are critical, there is a present need for such an optical collimating apparatus which is both compact and light-weight.